This invention relates to equipment for distributing and storing large quantities of asphaltic mixtures in a multiple silo or bin system. More specifically, this invention relates to a multi-silo distribution and storage system to effectively control emissions during normal plant operations and to minimize degradation of asphaltic mixtures through oxidation during periods of prolonged storage.
Several techniques and numerous equipment arrangements for the preparation of asphaltic compositions are known from the prior art. Continuous production of asphalt compositions can be achieved, for example, with a drum mixer asphalt plant. Typically, water-laden virgin aggregates are dried and heated within a rotating, open-ended drum mixer through radiant, convective and conductive heat transfer from a stream of hot gases produced by a burner flame. As the heated virgin aggregate flows through the drum mixer, it is combined with liquid asphalt and mineral binder to produce various asphaltic mixtures as the desired end-product. Optionally, prior to mixing the virgin aggregate and liquid asphalt, reclaimed or recycled asphalt pavement (RAP) may be added once it is has been crushed or ground to a suitable size. The RAP is typically mixed with the heated virgin aggregate in the drum mixer at a point prior to adding the liquid asphalt and mineral fines.
In earlier times in this industry, one of the bottlenecks in paving construction had been trucking the asphalt mixtures from the production plant to the job site. In order to decrease the trucking expenses and also the waiting time of a truck at the production plant, temporary storage and loading facilities for asphalt plants were developed. Initially, such facilities included an elevating conveyor, such as the drag slat conveyor disclosed in Rheinfrank, Jr. U.S. Pat. No. 3,647,047, to receive asphalt mix from the production plant and to deliver it to the top of a large cylindrical silo supported above a truck load-out area. This type of temporary storage operation successfully reduced truck waiting time since a full truck load of asphalt mix was available when a truck arrived from the job site. In addition, by smoothing out loading times and enabling the orderly delivery of asphalt mix from the plant to the job site, fewer trucks were required for paving construction.
It was found, however, that the asphalt mixtures trickling into the large storage bin from the elevating conveyor caused separation of the aggregate within the mix which tended to roll to the outside of the cone of material within the bin. This problem was solved in the early 1970's with the development of the batcher as disclosed in Rheinfrank, Jr. U.S. Pat. No. 3,777,909. The batcher sat atop the storage silo and collected the asphalt mix from the elevating conveyor. When the batcher filled, its contents were then discharged to the larger storage silo. Segregation of the aggregate rock in the mix was avoided by dumping the larger volume of material into the storage silo at one time. Today, a batcher is found on most all asphalt storage silos specifically for the purpose of preventing segregation of the aggregate.
Over time, as asphalt production techniques and efficiencies improved, greater storage capacities were needed both for storing larger quantities of material and for storing asphalt mixes of differing compositions for various job applications. Storage facilities with multiple bin systems resulted and a variety of distribution schemes have been proposed. These include a separate batcher for each storage bin, multiple conveyors to deliver material to the many bins, and moving conveyors. One such example is taught in Harris U.S. Pat. No. 3,182,859.
A recent and simplified solution to the problem of multiple bin distribution and storage utilizes a stationary batcher to receive asphalt mix from the production plant and a transport batcher to receive mix from the stationary batcher and to then travel on an indexing rail system to deliver its load to a preselected one of several storage bins. It is this particular type of multi-silo distribution and storage system to which this invention specifically relates.
The asphalt industry has traditionally faced many environmental challenges. The asphalt production plant characteristically generates, as by-products, gaseous hydrocarbon emissions (known as blue-smoke), various nitrogen oxides (NOx) and sticky dust particles covered with asphalt. Health and safety hazards resulted from the substantial air pollution control problems due to the blue-smoke produced when hydrocarbon constituents in the asphalt are driven off and released into the atmosphere. Within the asphalt production plant, exhaust gases are typically fed to air pollution control equipment such as a baghouse to filter particular matter from the exhaust gases. Thus, significant investments and efforts have previously been made by the industry in attempting to control emissions attributed to volatile hydrocarbon gases and particulates from the asphalt production plant itself.
Since the asphalt mix is typically delivered to the storage facilities as a hot or warm mixture, control of blue-smoke emissions from transferring equipment and from the storage bins themselves continue to be problematic. The asphaltic mixtures are exposed to atmospheric conditions when transferring mix from the stationary batcher to the transport batcher, and again when transferring mix from the transport batcher to the storage bin. The escape of volatile hydrocarbon emissions during these operational steps is inevitable.
Emissions also occur from the storage bin itself due to inadequate sealing of the upper access door of the bin and leakage around the lower discharge gate. During periods of prolonged storage of asphalt mixes, emissions from the storage bin continue as a result of the need for maintaining the storage material at elevated temperatures through various heating and/or insulation techniques. Volatiles from the mixes are therefore present and can escape around the lower discharge gate and upper access door of the storage bin.
Dillman U.S. Pat. No. 4,249,679 has proposed a seal for the lower discharge gate having a sliding door and a dispensing system to pump grease around the door and the discharge mouth of the storage bin. This was a cumbersome solution and was never widely accepted in the asphalt industry. Heretofore no solutions have been found to effectively and positively seal the upper access door of the bin.
In addition to the concerns over emissions during prolonged storage of asphalt mixes, degradation of the asphalt through oxidation has long been a problem. Clements et al U.S. Pat. No. 3,999,688 proposed an inerting system using tanks of carbon dioxide gas with a self-sealing top access door. However, the access door lacked any means to forcibly seal the upper opening and the inerting system was never widely adopted in the industry as a result of the maintenance issues associated with the need for an inventory of carbon dioxide tanks and frequent change out of the tanks in service.
A need remains in the asphalt industry for an improved multi-silo distribution and asphalt storage system to effectively control blue-smoke emissions, to provide positive seals for the upper access door and the discharge gate of a storage bin, and to provide a useful inerting system to minimize degradation of asphaltic mixtures through oxidation during periods of prolonged storage. The primary objective of this invention is to meet these needs.